The present invention generally relates to an external storage system used in an information processing system. More specifically, the present invention is directed to a storage control apparatus for controlling a data recording/reproducing operation for accessing a disk unit in which data is recorded in a fixed length block format by emulating a recording/reproducing operation for accessing a disk unit on which the data is recorded in a variable length block format.
In general, disk units are widely used as a data storage medium in information processing systems. There is a serious adverse influence caused by defects in the disk unit on the information processing system, and therefore it is very important to provide disk units having a high reliability.
One conventional approach to achieve a highly reliable storage unit is disclosed in, for example, JP-A-53-148928. In the storage subsystem for storing the data transferred from the central processing unit according to the conventional technique as described in JP-A-53-148928, the data to be stored is equally subdivided into segments whose quantity is smaller than that of the storage units employed by the storage subsystem. A check sum of the byte lengths identical to the segments is calculated by exclusive-OR-gating these subdivided segments, namely a parity is produced. The respective data segments and the parity segments are separately recorded on storage units which are different from each other in view of possible malfunction. Even when one storage unit would malfunction, the data which has been recorded on this malfunctioning storage unit could be reproduced, whereby a highly reliable storage control unit could be realized.
The above-described conventional technique corresponds to a highly reliable technique with employment of parity. As a consequence, the respective data segments and parity segments become the same byte number, namely fixed length data. It is preferable to record such fixed length data on the recording medium in accordance with the fixed length block format (fixed block architecture (FBA) format). On the other hand, since most of large-scaled information processing systems effectively utilize disk space, the data is normally recorded in accordance with such a variable length block format that the disk track recording format is divided, depending on the byte lengths of the records, namely the count-key-data (CKD) format. Thus, it is difficult to apply the above-described highly reliable technique to large-scaled information processing systems.
JP-A-1-306917 discloses one CKD emulation method for emulating the recording/reproducing of a disk unit on which data is recorded in the CKD format (referred to as a "CKD disk unit" hereinafter) by utilizing a disk unit on which data is recorded in the FBA format (will be referred to an "FBA disk unit" hereinafter). According to the conventional technique described in JP-A-1-306917, since the data recording to the disk unit is performed in the FBA format while maintaining the byte displacement relative positions of the CKD formatted records, the CKD formatted data is addressable. As a consequence, the highly reliable technique described in JP-A-53-148928 may be applied to an information processing system where data is recorded in the CKD format by way of this CKD emulation method. However, the CKD emulation method as described in JP-A-1306917 never considers defects or faults happening while the data is outputted to the disk unit. When the CKD disk unit is emulated by the FBA disk unit, the data boundary of the CKD formatted record is not coincident with that of the FBA formatted block. As a result, when an update request for the CKD formatted data is issued from the central processing unit, all CKD data containing data other than data portions corresponding to the CKD formatted record which should be updated, would be stored into the disk unit. Therefore, if defects happen to occur in the data writing operation to the disk unit, then there is a problem in that the CKD records other than the CKD formatted record which is required to be updated by the central processing unit, may be destroyed.
Meanwhile, the SCSI (Small Computer Systems Interface) ruled by ANSI (American National Standards Institute) has been proposed as a common interface capable of commonly utilizing various FBA disk units developed by several computer firms.
In this SCSI, both of an address and a transfer byte number of an FBA block which should be accessed are designated in a CDB (command descriptor block) in order to access data stored on a disk unit. Although accessing operations of the plural FBA blocks could be designated by a single CDB, the FBA blocks to be accessed must be logically continued with respect to each other. On the contrary, in the event of interface in accessing the CKD disk unit, a plurality of discontinued CKD records can be accessed by a single input/output request. Under such a circumstance, when the CKD emulation would be carried out by employing a SCSI as the interface of the FBA disk unit, a plurality of input/output requests must be issued to the disk unit. Furthermore, there are problems that not only a large overhead is required to issue such a CDB, but also a large latency may occur. Such problems may be solved by previously loading the data in the access range required by the central processing unit with respect to the data read request. However, the number of accessing operations to the disk unit cannot be reduced with respect to the data write request.